1. Field of the Invention
This invention relates to the preparation of zeolite/SiC composites and their use as a catalyst or a catalyst support for chemical reactions in the gaseous or liquid phase.
2. Description of Related Art
Materials with a zeolitic structure are used in many industrial processes as catalysts, and particularly in the petrochemical industry. The article “Zeolithes: Catalysts for Organic Syntheses” by W. Hölderich, M. Hesse and F. Näumann, published in Angewandte Chemie Int. Ed. Engl. 27, p. 226–246 (1988), contains a long list of chemical reactions that could be catalysed by zeolites. U.S. Pat. No. 5,994,603 (Exxon) shows the use of a zeolite-based catalyst for methylation of toluene.
However, due to the small size of the zeolite crystals, it is often difficult to use them directly, due to pressure loss phenomena in chemical reaction vessels and systems. The solution to this problem usually adopted consists of adding a binder to the crystals, usually a silicon oxide or aluminium oxide type of binder, and preparing extruded bodies. This method has the disadvantage that the binder is not chemically inert. Consequently, parasite reactions can occur under catalyst usage conditions. Furthermore, the binder may make part of the zeolite inaccessible to the reagents.
Research work has been carried out to overcome these disadvantages that has been aimed towards the synthesis of supported zeolite-type materials. The article “Growth of ZSM-5 films on alumina and other surfaces” by R. Lai et al, published in Microporous and Mesoporous Materials vol. 37, p. 9–19 (2000), describes growth of ZSM-5 type zeolite films on the surface of tubes or plates made of α-Al2O3, zirconia and quartz, as well as their use for gas separation.
Other articles deal particularly with the synthesis of supported zeolites for use in catalysis. The article “Zeolite coatings and their potential use in catalysis” by J. C. Jensen et al, published in Microporous and Mesoporous Materials, vol. 21, p. 213–226 (1998), describes different synthesis methods used and the different substrates (supports) most frequently used for the preparation of supported zeolite type composites: α-Al2O3 or γ-Al2O3 extrusions or pellets, monoliths of Si, TiO2, α-Al2O3, plates of stainless steel, quartz, pressed carbon, and glass, as well as α-Al2O3 foam, carbon fibres, vegetable fibres, steel wires, and even inert materials such as Teflon.
The use of silicon carbide supports for the deposition of zeolites is known in itself. Patent application WO 98/06495 (Technische Universiteit Delft) mentions the deposition of zeolites on SiC without giving any further information. The article “Preparation and catalytic testing of zeolite coatings or preshaped alumina supports” by N. Van der Puil et al, published in Microporous and Mesoporous Materials, vol. 27, p. 95–106 (1999), describes the synthesis of MFI structural type zeolites (ZMS-5, Silicalite-1) or BEA (Beta) type zeolites supported on α-Al2O3 pellets and extrusions. The use of SiC and SiC/Al2O3 as a support for the synthesis of such composites is also known.
The deposition of zeolites on porous ceramics is described in an article by G. B. F. Seiger et al, “In situ synthesis of binderless ZSM-5 zeolitic coatings on ceramic foam supports”, published in Microporous and Mesoporous Materials, vol. 39, p. 195–204 (2000). However, shaping of these materials is difficult, and they are fragile and expensive. The doctorate thesis “Development and catalytic testing of zeolitic coatings” by N. van der Puil, presented on May 13, 1997 at Delft University of Technology, Delft, Netherlands, describes the synthesis of beta zeolite supported on α-Al2O3 pellets and extrusions and on SiC grains. Patent application JP 06182214 A mentions the use of zeolite/SiC composites as a combustion catalyst support.
Most supports used in the past, such as silica (glass, quartz) or densified alumina have several other disadvantages; namely, they are not chemically inert, and in particular react with the zeolite synthesis medium by partially dissolving in it, such that the initial composition of the synthesis gel is altered. Furthermore, these supports have a very low specific surface area, typically of the order of 0.1 to 5 m2/g, such that the quantity of zeolite rarely exceeds 10% of the composite by mass. Finally, another big disadvantage of supports such as silica and alumina is that the zeolite adheres poorly to them and separates easily forming a powder. These disadvantages are a nuisance for the use of supported zeolites, particularly in the chemical industry, since they reduce the life of the catalyst and lower the efficiency and selectivity of the target reaction by direct loss of the active phase that can be drained by the gas or liquid to be treated. Furthermore, when the supported zeolite has at least partially disintegrated, the support may be exposed to the reactive environment; this support can then catalyse parasite reactions.
The problem related to poor adhesion has not been satisfactory solved. Some documents (see article by G. B. F. Seijger et al., the doctorate thesis by N. Van der Puil, patent application JP 06182214 A and patent application WO 98/06495 mentioned above) describe the use of chemically inert supports, and particularly SiC. However α-SiC, often called “Non-porous SiC” was used in all these cases. As described by N. Van der Puil in his doctorate thesis, the anchor force of zeolite crystals at the support surface is very small and part of the zeolite is detached during post-synthesis treatments carried out to obtain the composite in its final form. Several solutions have been proposed to overcome this problem.
Seijger et al (see the previously mentioned article) have succeeded in depositing an important charge of zeolite on an SiC/Al2O3 ceramic foam. However, the support is embrittled during synthesis and the composite synthesized by such a method is brittle.
Patent application JP 06182214 A describes another method involving calcinating SiC at high temperature to form a layer of SiO2 at its surface. If the support thus prepared is subsequently put into the presence of an aqueous solution containing an aluminium source under hydrothermal conditions, the silica layer will be transformed into zeolite. This composite is then used as a catalyst support to carry out combustion reactions. Although this method leads to strong interaction between the zeolite and the support, the zeolite charge remains low because only the surface layer composed of SiO2 can be transformed.
Patent application WO 98/06495 presents another support preparation method, whereby the support is put in the presence of a solution containing a structuring material of the same nature as that used to prepare the zeolite synthesis gel. After the gel has been prepared, the support is brought into the presence of the synthesis gel under hydrothermal conditions. This method can give a good anchorage of the zeolite on an α-Al2O3 support.
In each case in which SiC was designed as a support for a zeolite-SiC type composite, only α-SiC which is non-porous and therefore has a low specific surface area (<5 m2/g) was used. It has been demonstrated (see previously mentioned documents) that the above method may often produce one or more of the following disadvantages, that either (i) the zeolites are only weakly anchored to the SiC surface, or (ii) that the synthesis conditions make the final composite brittle, and/or (iii) that the synthesis only creates a very thin layer of zeolite on the support, such that the efficiency is always low. These various disadvantages all contribute to the result that those skilled in the art have now abandoned the use of SiC, and instead typically use α-Al2O3 for the synthesis of such composites.